May I ask how folks are gauging the chain line measurement?
The boy at the back there will instantly cease asking difficult questions!
The first thing is not to tie yourself up in knots trying to calculate everything. There's an example on this board where several people tried to help me in my aspiration of achieving a perfect chainline, with a permanent record of all the blind alleys.
In fact, it is real simple. From Thorn you know the diameter of your seat post. Take half of that and write it down. Don't try to measure from some notional centre of the seat post. The maker of your chainring tells you how thick it is. Write down half that thickness. From Rohloff you know the centre-line of your sprocket, which is the chainline you're trying to achieve. Now with a builder's steel rule, the kind that measures right to the edge of the metal, measure from the drive side edge of the seat post to the outside edge of the chainring. This measurement plus half the seat tube width minus half the chainring thickness is your starting point. the difference between this measure and the sprocket distance from the centre of the frame aka Rohloff's perfect chainline is the amount you want to move your chainring.
If the existing chainline is within 1mm, congratulations, the job is done. Unless you're a time-served mechanic, you're not likely to do better and the possibility of doing worse must be considered.
Under Rohloff's latest advice, even a few millimetres off will do the gearbox no harm, but may wear your sprocket and chainring and chain a bit faster than a perfect chainline.
If you have to adjust it, or want to, to make your components last a bit longer, read Martin's post for ways and means. All that I do differently is the order in which I approach the job, merely for convenience and the least amount of spannering (American: wrenching) for fear of upsetting some other relationship that I may not even know about.
First off, if you have one of the common so-called "compact" cranksets, there is no law that says you need to bolt your single chainring onto a particular set of threaded holes; you can move the chainring in or out a quite big chunk if you need a quite big chunk of realignment. Second adjustment, still fairly big: Rohloff supplies or at least supplied when I bought my Rohloff bike, with every bike a set of chainring spacers. I don't know if Thorn supplied these "free" spacers with the bikes or whether they took the view that they didn't want amateurs messing with a properly set-up bike, but you can buy them. Now you should be two or three millimeters away from a straight chainline, and can call it a day unless you're obsessive about perfection.
If you want to continue, you will need to remove at least the cranks and possibly the bottom bracket. First off, in theory, though I've never seen such a thing, you can buy an asymmetrical bottom bracket with unequal arm lengths to arrange the correct chainline without disturbing the tread-width, what the smart cyclists now call the Q Factor. You may also arrange your bottom bracket asymmetrically in the BB shell on the bike, in the case of your bike inside the eccentric bottom bracket carrier, to arrange a little more or less length of axle showing on the drive side; this would ba a small amount and would unbalance your tread width, which old roadies claim to be sensitive to. (I believe them, not because I'm that sensitive about where I put my number twelves, but because I'm extremely sensitive to putting my hands and arms in positions critical to 1mm in all directions to avoid buying my physio's teenage children a BMW convertible each.) Third, you can use spacers on the axle for fine-tuning. These are just washers of warning thicknesses, technically called shims. I used, from memory, a 1.6mm (what I had v. what was required for perfection) to make my final adjustment, plus a 2mm on the other side to obtain clearance from the frame for said size twelves, necessary because I cycle in street clothes including sturdy street shoes, and don't want to damage the irreplaceable heritage paintwork on my bike. I happened to have a can of spray paint that matched my bike colour (non-RAL "forest green", a custom colour mixed in The Netherlands, so I used that on the spacers to make them invisible, but plain matt black will also do the job.
That brought my chainline to less than 1mm from perfectly straight -- or even zero offset, actually perfectly straight, because 1mm is probably within the margin of measuring error. You'd need sophisticated fixtures bolted to the bike frame to make manual measurements down to say half a millimetre on a bike, and I'm not that obsessive. After that, you're into lasers and diminishing returns.
It's the sort of job you do only once unless you churn components all the time. The process last happened when I converted the bike to a bottom bracket shell-mounted central electric motor, probably a decade ago, so E&OE.
